Apep is the Ancient Egyptian god of Chaos.
Apep has been one of the biggest enemies of the forces of Good since time immemorial. However, Judson and Charlene managed to seal him in a sarcophagus, centuries ago. He was released at the beginning Season 3, and tried to use the The Ankh to release Pure Evil from its prison.
As the god of Chaos, Apep is short-tempered, vicious, and stops at nothing to accomplish his goals.
Posession: Apep needs a vessel in order to manifest in the human world. Without said vessel, he is powerless, and looks similar to a swarm of bugs.
Rage Inducement: Apep is able to throw make nearby humans fight with each other for no reason. However, friends who have set their differences aside, such as The Librarians, are completely immune to his powers.
Manipulation: Apep is able to perfectly pose as the person he is possessing until he wants to reveal himself. He was able to fool The Librarians many times.
Real Life Counterpart
Apep, an entity represented as a huge snake, is the embodiment of chaos and sworn enemy of all forces of truth, good, and order, according to Egyptian Mythology. Every night, he fights fiercely with his archenemy Ra, attempting to devour him. If Apep succeeded, night would become eternal, and evil would conquer the world.